1. Field of the Invention
The present invention relates to a data reproduction method, a data reproduction apparatus, an optical disk, and a data recording/reproduction apparatus, and more particularly to a data reproduction method, a data reproduction apparatus, an optical disk, and a data recording/reproduction apparatus for reproducing data recorded in an optical disk.
2. Description of the Related Art
Along with the advances in digital technology and data compression technology, optical disks such as DVDs (Digital Versatile Disc) are gaining greater attention as media for recording data, such as music, movies, photographs, and computer software. Thus, as optical disks become less expensive, optical disk apparatuses (information reproducing apparatuses) for reproducing data recorded in the optical disks have become widely used.
In an optical disk apparatus, a light beam for reproduction purpose (reproduction light beam) is condensed to an optical disk, to thereby reproduce information based on the light reflected from the optical disk (See, for example, Japanese Laid-Open Patent Application No. 2002-319137).
In recent years, there has been proposed an optical disk (hereinafter referred to as “super resolution optical disk”) that allows reproduction of data from recording marks having a pitch smaller than a diffraction limit (hereinafter referred to as “super resolution reproduction”) (see, for example, Japanese Laid-Open Patent Application Nos. 6-183152, 5-205314, 11-250493, and 2001-250174). The super-resolution optical disk has, for example, a layer (hereinafter referred to as “super-resolution layer”) containing a material whose optical constant (e.g., refraction index real part n and refraction index imaginary part k) changes when light is condensed thereto.
Accordingly, when a reproduction light beam is condensed to the super-resolution layer, a fine mask area or a fine aperture area is formed inside the beam spot of the reproduction light beam in accordance with the change of the optical constant, to thereby achieve high resolution reproduction of data.
However, in a case of attempting to reproduce data recorded in the super-resolution optical disk by using the optical disk apparatus shown in Japanese Laid-Open Patent Application No. 2002-319137, phase distortion tends to occur in an RF signal obtained from reflected light. This causes reproduction error to occur frequently.
Accordingly, Japanese Laid-Open Patent Application No. 1996-221839 discloses a method using a waveform equalizer in a reproduction system using a slicer for performing phase correction. With this method, asymmetry of reproduction signals due to super-resolution reproduction of a magneto-optical disk can be corrected.
However, since the method disclosed in Japanese Laid-Open Patent Application No. 1996-221839 does not use a PRML (Partial Response Maximum Likelihood) method which improves performance of decoding recording marks of small pitch, it is difficult to achieve high density even where super-resolution reproduction is used.
Optical aberration from tangential tilt may cause phase distortion of reproduction signals also for ordinary optical disks. Accordingly, Japanese Laid-Open Patent Application No. 2002-32919 discloses a method of using a PR (Partial Response) filter in a PRML method for phase correction.
More specifically, Japanese Laid-Open Patent Application No. 2002-32919 disclosed a method of using an adaptive PR filter as a PR (Partial Response) filter that provides a predetermined intersymbol interference.
This adaptive PR filter includes a FIR filter having a digital configuration. In order to achieve high precision of waveform equalization, reproduction signals are provided to the adaptive PR filter at clock timings after clock extraction of the reproduction signals. Accordingly, phase correction can be performed without being affected by, for example, rotation inconsistency. Thus, waveform equalization can be performed with accurate PR characteristics.
However, compared to the super-resolution method using magnetic transfer of a magneto-optical disk or asymmetry of a beam spot caused by tangential tilt, super-resolution reproduction causes a considerable phase distortion. Therefore, it is difficult for adaptive signals to have clocks extracted beforehand for achieving the function of phase correction.
Furthermore, from another aspect, in recent years and continuing, there is a growing demand for an optical disk with greater data capacity along with the advances in digital technology and improvement of data compression technology. As for methods of satisfying such demand, there are, for example, a method of reducing the beam spot diameter of a laser beam used for data reproduction and improving the resolution of an optical system.
For example, in an optical disk apparatus used for reproducing and recording data with an optical disk (e.g., Blu-ray Disc) having greater data capacity than a DVD (Digital Versatile Disc), data can be read out and recorded with a recording mark having, for example, a diameter no greater than 0.160 μm-0.138 μm by condensing a laser beam having a wavelength of approximately 390 nm-420 nm to an objective lens having a numeric aperture of approximately 0.70-0.90 and focusing the laser beam on a recording layer of the optical disk with a beam spot diameter of approximately 0.48 μm.
However, due to factors such as transparency of the polycarbonate material used in the optical disk, it is becoming more difficult to provide a laser beam with shorter wavelength or an objective lens with higher numeric aperture. Therefore, in recent years, there has been proposed an optical disk (super resolution optical disk) that allows reproduction (super resolution reproduction) of data from recording marks having a pitch smaller than a diffraction limit (see, for example, Japanese Laid-Open Patent Application Nos. 6-183152 (Patent Document 1), 5-205314 (Patent Document 2), 11-250493 (Patent Document 3), and 2001-250174 (Patent Document 4)). The super-resolution optical disk has, for example, a super-resolution layer containing a material whose optical constant changes when a laser beam is condensed thereto. Accordingly, when a laser beam for data reproduction (reproduction laser beam) is condensed to the super-resolution layer, a fine mask area or a fine aperture area is formed inside the beam spot of the reproduction laser beam in accordance with the change of the optical constant, to thereby achieve high resolution reproduction of data.
However, in a case of attempting to reproduce data recorded in the super-resolution optical disk by using the optical disk apparatuses shown in Patent Documents 1-4, phase distortion tends to occur in an RF signal obtained from reflected light. This causes reproduction error to occur frequently. Hence, it is difficult to increase data capacity with the optical disk apparatuses shown in Patent Documents 1-4.
In recent years, there has also been wide use of an optical disk apparatus having a digital data reproduction apparatus using a PR (Partial Response) method. The use of such optical disk apparatus is due to the growing difficulty of reading a single bit of digital data without encountering interference of a neighboring bit (intersymbol interference) along with the increase of recording density of the optical disk (recording medium) to which the digital data are recorded.
The partial response method is used to prevent deterioration of signal characteristics during a equalization decoding process by actively generating predetermined linear waveform interference. Recently, a PRML (Partial Response Maximum Likelihood) method, which is a combination of the partial response method and a ML (Maximum Likelihood) method, has been used for performing high precision signal processing.
However, the reading system for reading data from the optical disk has a non-linear property owing to the principle of reading out signals by using light diffraction. Thus, an RF signal has an asymmetric property caused by the non-linear property of the reading system. Furthermore, the RF signal also includes a non-linear component created by changes in the position of a recording mark of a recording pattern. Such change in the position of the recording mark is caused by temperature interference during a data recording operation. The asymmetric property and the non-linear component of the RF signal make it difficult to increase the density of recording data to the optical disk.
Accordingly, Non-Patent Document 1 (“Adaptive Partial-Response Maximum-Likelihood Detection in Optical Recording Media”, Naoki Ide, ISOM2002) proposes a maximum likelihood method using a non-linear compensation table with consideration of intersymbol interference of the non-linear component. However, since the maximum likelihood estimate bit length and the non-linear compensation length is the same in the method disclosed in the Non-Patent Document 1, the non-linear compensation effect is limited in a case where the non-linear compensation length is short with respect to the length of the beam spot on the optical disk (recording medium) (i.e. in a case of high density recording). Particularly, the non-linear compensation effect is insufficient during reproduction of data from a high resolution optical disk in which data are recorded with a density greater than the diffraction limit of the optical system or during generation of aberration (e.g., coma-aberration, astigmatism) where the beam spot diameter is increased.
Accordingly, as shown in, for example, Patent Document 5 (Japanese Laid-Open Patent Application No. 2004-326839), there is a method of performing non-linear compensation and maximum likelihood estimation by anticipating linear intersymbol interference of a predetermined bit length and using a non-linear compensation table with a longer bit length. However, since the method disclosed in Patent Document 5 requires the maximum likelihood estimation to be performed with the same range as the non-linear compensation bit length, the circuit size becomes considerably large as the non-linear compensation range is increased. In order to perform maximum likelihood estimation with this method, the status number is to be doubled whenever the bit length is increased by a single bit. Thereby, the circuit size is doubled. Furthermore, in a case where the number of pattern compensation bits is added three bits at a time, the circuit size becomes ten times greater or more. This causes the reproduction apparatus to become expensive.
Furthermore, Patent Document 6 (Japanese Laid-Open Patent Application No. 2001-126394) discloses a method of performing non-linear compensation on a long range bit string without increasing the circuit size by using results of previous (past) provisional determination results. However, the increase of recording density by using the non-linear compensation is small (10%-20%). Therefore, this method is insufficient for achieving a significant increase of data capacity.
Furthermore, from yet another aspect, in using the partial response method, noise can be reduced and bit error rates can be improved by selecting a partial response characteristic matching with the characteristic of the reproduction system being used. For example, Patent Document 7 (Japanese Registered Patent No. 3696130) discloses a signal processing apparatus having partial response characteristics that have a symmetric shape where the origin (0) is the center (e.g., PR (a, a), PR (a, b, a), PR (a, b, b, a), PR (a, b, c, b, a), PR (a, b, b, b, a), PR (a, a, b, a, a) in a case where the PR characteristics can be expressed with 5 bits (“a”, “b” “c” each being a given real number)). However, the symmetrically shaped partial response characteristics of Patent Document 7 are far from matching with the characteristics of the analog reproduction signals read out by irradiating an asymmetric beam spot to the super resolution optical disks disclosed in Patent Document 1-4. This results in a problem of inconsistency (mismatch) between the characteristics of the reproduction system and the partial response characteristics. This leads to increase of bit error rate.